A quick Flash change Idea
I don't believe flash is grossly overpowered; I just believe its too safe and has no counterplay to it. I've seen other people complain about it and I have an Idea to help it not be quite as strong.
Base effect is fine, leave it be, but add a extra effect to it. Have the distance be based on Bonus or negative movement speed.
like say up to +/-33% range modified by 1% per movement speed increase or decrease. It would allow for a little more distance If you use flash smartly, to avoid things, but let you still be punished a little for say running into a thresh's box